Telephone transmitter



A. E. KEITH.

TELEPHONE TRANSMITTER. APPLICATION FILED APR-14. 1912.

I 1,41 2,080. Patented Apr. 11, 1922.

II I ALEXANDER KEITH, or HIivsDALn, "ILLINOrs, hssienon 'T'C AUTQMATIG 'nr no'rnr'c r COMPANY, on CHICAGO, ILLINoIse-coeizoRAmrou or ILLINOIS.

TELEPHONE TRANSMITTER- T 0 a ll w izlo m it may concern Be "it known that I, ALEXANDER E. KEIT a citizenof the .United States of America, anfd'resid'ent of Hinsdale, Du Page County, Illinois, have invented certain new and useful Improvements in Telephone Transmit ters, of whiclr'the following is a spe'c'ilicatio n. T v M. 1' invention, relates to 'telephone transmitters and hasfor its object to provide a microphone structure in which the detrimental effects due to an inactive mass of granulated carbon or other comminuted material is eliminated; and in which the p0ssibihtvof the occurrence commonly known as,

packing, is reduced to a minimum. 1 I f In the-ordinary type of transmltter eniploying that ithas'settled in the base of thechamber and 15 only slightly agitated .the movement er the diaphragnr; This mass. is of relatively low. resistance and forms a shunt tofthe pathwhose resistance is actually sub- 'ject to variation; therebyreducing the 21111 plitude of thecurrentfluctuations. J :lnaccordance with my invention one of the electrodes, preferably the rear or fixed ,elec: erode, is'inade considerably smaller in size than the other electrode, ivitlr'the result that any current passing through the inactive material must also pass through the material undergoing. the maximum agitation. Although the average current values must necessarily be' 'decreasedo somewhatflon account of, the smaller area of one-of the electrodes, the range of transmission remains the same oi-is even 'in's'onie cases increase d because-of the increasedamplitude of tlie c urrent variations, whlch produces clearer and more distinct articulation than has hith-' erto'be'en'kn'own. The packing of the granu lated carbon is prevented by so constructing the containing chamber and electrodes that atilea st {onefend 0f thefeha'mber, and. both ei 'dsrr y, e P y smwth'phne surfaces. In other words at least one and preferably both of the electrodes areso designed that they-doinot project intothe carbon chamber, leavingarecess around their edges in which. the carbon can pack; but on the; contrary the opposite faces of the electrodes .areperfec'tly flush with the end Wall ofthe chamber of which they form a part.

granular carbon as a resistance vary- V I lng material there is a. portion of the carbon which is-practically inactive due to the fact Specification of Letters Patent, atginfed Ap g 11 1922; npplicationcfiieaprn14,1917. Serial 1\To 61',94 7. r

had to the accompanying drawing, in which i Fig. (his a back View eta-complete transmater; Fig. 2 is a cross section (on the liner l, showing .a'microphone construction involving a; flush type rear electrode and a standard frontelectrcde; while Fig. is a modification, of Fig. 2 in which bothrear and front electrodes are of the flushlt ype; V 1 Referring to Figs; landQ ofthedrawing the reference character 1 designates the usual mouthpiece screwed into a threaded opening in the faceplate 2.", The: face plate :1

2 is provided with a rearwardly projecting circular :rim 5 within which is driven-by special. machinery "and held by frictiona heavy flanged ring seenmost clearly in .Fig. 1. The metal bridge piece 3 isformed .4.

integrally with the ring 4: and actsas a-support for the elements of the microphone.

This latter comprises the :carbon electrodes 8 and 2 1fand the 'chamberfi partially filled with thecomminuted carbon 11. The rear electrode8 is secured to a metal disk"? which 1s provided with a central screw threaded stem 9. The chamber 6 .iscsimply a; plain" cup drawn preferably from: brass, is lined with, insulating materiahpandi is, provided r with a central aperture in the bottom; The

stem 9 passes through'this' hole and a corresponding hushed aperture-in the bridge -3, and the nut 10 is screwed down to clamp the wholetogethei'. The chamber 6 and the nut are preferably insulated from the bridge .3 by means of washers of empire cloth} gIn order to fill the space between I the fsidesof the rear electrode-and the walls of the CllEiHl? be/r6, a washer provided, which maybe p of card board or other lnsulatin-g naterlal.

The. outside diameter: of the washeris evidently equal to the inside diameter of the cliamben'while thesdia meter of the hole in the ce'nter equaltothediameter of-therear f electrode The washer should fit tightly-and should beef suchthicknessthat when-in posit-ion twil'l form w th'the rear electrode a a plane surface-closing ,the rear end ofthe' chamber. It is desirablefthat this surface be smooth and polished.

through which is passed the stem 22.

1 and front electrodes.

'on over the rim of the chamber 6 andthe mica diaphragm is securely held between the edge of the chamber and the said ring.

The usual metal diaphragm 2O rests against an insulating ring 19 on the back of the face plate 2, and has at the center a hole y means of nuts 17 and 18 the stem 22 is rigidly secured to the diaphragm. The clamping spring comprising two members 23 and 24 is attached to the ring 4: by means of screws 25 and 26. The extremities of these spring members 23 and 24 press against the diaphragm 20 as shown in Fig. 2, there being a felt washer interposed between the diaphragm and the end of each spring.

The foregoing describes briefly one embodiment of my invention. By referring to the drawing it will be clear that owing to the relatively small diameter of the rear electrode any current" flowing through the comparatively inactive material in the bottom of the electrode chamber must also pass through the active material behind the front electrode, whereby all shunting effect is eliminated. 't willbe clear also that owing to the smooth construction of the rear wall of-the electrode chamber involving the flush type rear. electrode, the comminuted carbon will be free to risein the chamber when compressed, whereby the tendency to pack is greatly reduced.

In Fig. 3 is shown a modification in which the advantages pointed out above are obtained to an increased extent b em lo in the flush type construction for both the rear In this figure the microphone only is shown, together with a fragmentary portion of the bridge 3 and the main diaphragm 20. All parts except the frontelectrode and attachments are the same as corresponding parts in Fig. 2 and hence need not be described further. Some reference characters have been attached however, to aid in identifying the parts.

are desirably gold plated to avoid corrosion; or as an alternative, the electrode may be faced with platinum, foil after it and the bolt are assembled. The inner surface 48 of the flexible portion of the member 4.0 is painted with insulating paint or enamel of some kind in order to confine the conducting sur-' is obtained which embodies the advantages hitherto pointed out to the greatest possible extent. I

Having described my invention, what I consider to be new and desire to have protected by Letters Patent will be more specifically-pointed out in the appended claims.

What I claim as my invention is:

1. In a telephone transmitter, a cylindrical chamber containing resistance varying 'material, structures having oppositely disneed arallel lane surfaces closin the l P P a ends of said chamber, a portion only of each surface being of conducting material, the opposite conducting surfaces thus formed comprising the two electrodes of the transmitter,

a diaphragm responsive to air vibrations for actuating one of said electrodes to vary the pressure on said resistance varying material, and an air space in said chamber above the said resistance varying material whereby it is permitted to rise and fall as the pressure is increased and diminished.

2. In a telephone transmitter, a cell or chamber containing comminuted resistance varying material, structures having parallel, plane inner surfaces forming the opposite ends of said chamber, each surface being composed part of insulating and in part of conducting'material, means for making electrical connections to the two opposite conducting surfaces, whereby they act as electrodes for the transmitter, means for actuat ing one of said electrodes to vary the pressure on sa1d comminuted material, and an air space in said chamber above the said 112- sistance varying material whereby it is permitted to rise and fall as the pressure is increased and diminished.

3. In a telephone transmitter, a cell 0 chamber containing comminuted resistance varying material, structures having parallel plane inner surfaces forming the opposite ends of said chamber, each surface being composed in part of insulating and in part of conducting material, the conducting portion of one surface being a substantially greater portion of the whole than the conducting portion of the other surface, and means for making electrical'connections to the two opposite conducting surfaces whereby they act as electrodes for the transmitter.

4. In a telephone transmitter, a cylindrical chamber containing a resistance varying material, an electrode at each end" of the chamber having a plane-surface the area of which is less than the area of a cross section of the said'chamber, oppositely disposed rings of insulating material cooperating with said electrodes to form opposite plane surfaces closing the ends of said chamber,

means for actuating one of said electrodes;

to vary the pressure on said comminuted material, and an air space in said chamber above the said resistance varying material whereby it is permitted to rise and fall as the pressure is increased and diminished.

v 5. In a telephone transmitter, a chamber containing a resistance varying material, an electrode, at each end of the chamber having a plane surface partially closing the chamber, insulating material cooperating with each electrode and so disposed as to form in conjunction with the surface of the electrode a substantially plane surface completely closing the chamber, means for actuating one of said electrodes tovary the pressure on said comminuted material, and an air' space in said chamber above the said resistance varying material whereby it is permitted to rise and fall as the pressure is increased and diminished. I

6. In a telephone transmitter, a chamber containing resistance varying material, a fixed electrode at one end of said chamber, insulating material cooperating with said electrode to forma plane surface closing one end of the chamber, a movable electrode suit 7 i brations for actuating said movable elec j trode to vary the pressure on sald resistance varying material, and an air space in said chamber above the said resistance varying material whereby it is permitted to rise and. fall as the pressure is increased and diminished.

fixed electrode at one end of said chamber, insulating -material cooperating with said electrode to form a plane surface closing one end of the chamber, anda movable electrode suitably supported at. the otherend of said; 1

chamber, the said movable electrode having a substantially greater exposed conducting surface than the said fixed electrode. I

8. In a telephone transmitter, a chamber containing, resistance varying material, an electrode at one end of thechamber of smaller diameter than the saidchamber, an 7 insulating ring filling the space between the A edge of the electrode and the wall of" the chamber and cooperating with the electrode to form a plane surface closing 'onefend of the chamberQa movable electrode at the'opposite end of the chamber, a diaphragm responsive to air'vibr'ations for actuating said movable electrode to vary the pressure on 7 said resistance varying material, and an air space in said chamber above the said resist- I I ancevarying material whereby it is permitpressure is inted to rise and fall as the creased and diminished. I

Signed by me at Chicago, Cook State of Illinois, this 9th day of April 1917 ALEXANDER E. KEITH.

4 County, 

